Immune cells have been traditionally considered responsible for mediating IBD, but recent evidence indicates that other cell types are also actively involved in IBD pathogenesis. In particular, the lymphatic system is closely associated with the immune response by providing an extensive network of vessels responsible for traffic of T cells, B cells, macrophages and dendritic cells and delivery of antigens to lymph nodes, where generation of effector cells takes place. In view of this vital function the lymphatic system is now considered an intrinsic component of the immune system, and represents part of the body's immunological surveillance and defense system. In the resolution phase of an inflammatory response the lymphatic system is charged with the removal of accumulated immunocytes, antigens, and inflammatory mediators away from inflammatory foci, so that tissue homeostasis can be re- established. To perform these crucial functions the lymphatic system undergoes lymphangiogenesis, a growth process that is beneficial when properly regulated but, if persistent or dysregulated, results in impaired lymphatic drainage and contributes to exacerbate inflammation. Considering this, it is intriguing to remember that IBD, and CD in particular, has for a long time been associated with obvious alterations of intestinal lymphatics. Dilated lymphatic channels, lymphatic obstruction, and formation of ectopic lymphoid tissue are well recognized features of IBD, but until now no attention has been given to what these abnormalities might mean for disease pathogenesis. We have recently reported that the lymphatic system plays a role in murine colitis, and we have succeeded in isolating and culturing human intestinal lymphatic endothelial cells (HILEC). Therefore, with these new tools we propose to test the following central hypothesis: Lymphangiogenesis is a critical component of IBD and contributes to disease pathogenesis. This hypothesis will be tested by the following specific aims: 1) Analysis of the lymphatic vasculature in human and experimental IBD;2) Assess the function of lymphatic endothelial cells in human and experimental IBD;3) Investigate the role of lymphoid tissue inducer (LTi) cells in IBD-associated lymphangiogenesis;4) Explore the potential therapeutic effects of modulating the lymphatic vasculature in experimental IBD. Improving our understanding of the lymphatic system in gut inflammation and developing interventions aimed at restoring normal lymphatic function will have a positive impact on scientific knowledge and may foster novel therapeutic approaches for IBD. PUBLIC HEALTH RELEVANCE: The proposed studies aim at understanding the function of the lymphatic system in patients with inflammatory bowel disease (IBD). The lymphatic vessels are similar to blood vessels but, instead of carrying red blood cells, white blood cells, oxygen and nutrients throughout the body, they carry the bulk of the immune cells responsible for the body's defense. Therefore, the function of the intestinal lymphatic system is closely related to the function of the gut immune system, and any abnormality of lymphatic function may impair the local immune response and cause gut inflammation. There are reasons to believe that the gut lymphatic system is overly extended in IBD patients, but its capacity to carry immune cells away from sites of inflammation is deficient. If so, an excessive accumulation of immune cells occurs in the tissue causing inflammation to linger, as typically seen in IBD patients. A series of experiments are therefore propose to understand why the lymphatic system fails to work efficiently in the gut affected by IBD, and what can be done to restore its normal function and eliminate inflammation.